1. Field of the Invention
This invention relates to refrigerating machine oils, more particularly to such a refrigerating machine oil suitable for a refrigerating machine using ammonia as a refrigerant.
2. Description of the Prior Art
Due to the recent issues concerning with the ozone shield depletion, conventional refrigerants for refrigerating machine such as CFC (chlorofluorocarbon) and HCFC (hydrochlorofluorocarbon) have been targeted for regulation. In place of these refrigerants, HFC (hydrofluorocarbon) has been used as such a refrigerant. However, since the HFC refrigerant also has a problem that it is high in Global Warming Potential (GWP), it has been considered to use refrigerants containing natural materials as alternative refrigerants for the fluorocarbon type refrigerants.
Conventionally, ammonia has been used as a refrigerant for the industrial use, and mineral oils have been used as refrigerating machine oils for use with an ammonia refrigerant. However, due to inmiscibility of ammonia with mineral oils, it is rather difficult for the oil pumped out from a compressor to return to the compressor through the refrigerating cycle, resulting in poor lubricity in the compressor and the reduction of efficiency of heat exchange. Under these circumstances, the development and research of a refrigerating machine oil miscible with ammonia has been progressed.
When ammonia is used as a refrigerant, water possibly enters into a refrigerating cycle due to the hygroscopicity of ammonia itself which is extremely high, compared with fluorocarbon type refrigerants. When a refrigerating machine oil containing a mineral oil is used, the water entering into a refrigerating cycle creates a problem that the water separated from the oil freezes and closes the line of the refrigerating cycle, which adversely affect the stability of the refrigerant and oil and of the pipings of the system. Therefore, a refrigerating machine oil for use with an ammonia refrigerant is required to be stable in the presence of water.
A study has been placed on a PAG (polyalkylene glycol) compound as disclosed in Japanese Patent Laid-Open Publication No. 5-009483 to use as a refrigerant which is miscible with ammonia. An oxyethylene oxypropylene copolymer has been regarded as being superior in miscibility and fluidity at low temperatures.
However, the use of PAG containing an oxyethylene group in its molecule poses a problem in terms of stability when water and oxygen enter into a refrigerating cycle. For the foregoing reasons, it has not been accomplished to develop a refrigerating machine oil for use with an ammonia refrigerant which has satisfyingly required properties such as lubricity, miscibility with a refrigerant, fluidity at low temperatures and stability, in a well-balanced manner.
In view of the foregoing, it is an object of the present invention to provide a refrigerating machine oil which can meet all of the requirements such as lubricity, miscibility with a refrigerant, fluidity at low temperatures and stability, in a well-balanced manner when used with an ammonia refrigerant.